The discharge of a RF excited CO2 slab laser is typically diffusion cooled. In diffusion cooled CO2 slab lasers, the excited (i.e., hot) CO2 molecules collide with the surfaces of the metal electrodes and become de-excited (i.e., cooled) by the collision and the transition to the ground state. In slab lasers, diffusion cooling is facilitated by providing a small separation (i.e., gap) between the parallel facing electrodes and by having the electrodes liquid cooled. The small gap ensures good heat transfer (i.e., a large number of collisions) and, therefore, cooling from the gas discharge to the metal electrodes. The liquid cooling flow in contact with the electrodes conducts heat away from the electrodes. The optimum gap dimension is determined by the gas pressure, RF excitation frequency, gas composition, etc. Keeping the discharge cooled is important because the output power of a CO2 laser is inversely proportional to the temperature of the discharge. The cooler the discharge, the higher the laser's efficiency.
The electrodes and the laser tube housing that contains the laser's electrodes, gas mixture and optical resonator are typically fabricated from Aluminum due to cost, good heat conduction and low weight. Since Aluminum corrodes in contact with water, a chemical stabilizer is usually added to the water to prevent corrosion within the Aluminum cooling passages. Copper is a preferred cooling material, but it cannot be used within the hermetically sealed laser tube housing because it oxidizes, thereby reducing the O2 content of the laser gas mixture. As the O2 content is depleted by the Copper, the laser power decreases. This problem is usually solved by plating the Copper surfaces exposed to the laser gas mixture within the laser tube housing with an inactive material such as Nickel.
In order to facilitate placing the electrode assembly into the long rectangular or round laser tube housing, which typically has narrow openings at each end, and to facilitate making the liquid cooling connections between the electrodes and a liquid coolant supply located outside the long laser tube housing, the electrodes are typically inserted at one end of the housing and then hermetically sealed within the housing with an Aluminum end flange using an O-ring. See U.S. Pat. No. 5,123,028, entitled “RF Excited CO2 Slab Waveguide Laser”, issued Jun. 16, 1992 (Ref. 1). Having the electrode coolant pass through one of the end flanges presents a serviceability problem because the mirror(s) of the laser's optical resonator are mounted on the end flanges, one of which is also holding one end of the electrode assembly in place. Consequently, inspection of the optical resonator's mirrors requires the disassembly of the coolant connections. The present invention addresses this serviceability problem by providing coolant connections to the electrodes through one of the sides and close to one end of the metal laser tube housing.
Even though most of the heat generated by the laser discharge is carried away by the coolant flowing through the electrodes, the gas contained within the hermetically sealed laser tube housing gradually heats up, thereby heating the laser tube housing. Since the mirrors of the optical resonator are typically mounted directly on the metal flanges, which are in turn directly mounted on the ends of the metal laser tube housing, the expansion and contraction and twisting and bending of the laser tube housing with changing temperature can negatively affect the laser's output performance (i.e., misalignment of the optical resonator can cause output power and beam pointing variations). The long temperature stabilization time of the laser tube housing compounds the variation problem when the laser is operated at different pulse repetition frequencies (PRFs). It becomes increasingly more difficult to ignore the effect that temperature variations within the laser tube housing have on the laser's output power as the output power increases.
In the prior art, the effects of the slow temperature drift on the laser's optical resonator caused by heating of the laser tube housing was either ignored, as in above-cited Ref. 1, or addressed by placing the mirrors on stiff Invar rods. Invar is well known to have a very low temperature coefficient of expansion and it has often been used to construct stable optical resonators. See U.S. Pat. No. 5,502,740, entitled “Stripline Laser Resonator”, issued on Mar. 26, 1996 (Ref. 2) and U.S. Pat. No. 5,278,859, entitled “Stripline Laser”, issued on Jan. 11, 1994 (Ref. 3). The Invar rod approach has its drawbacks because of the cost of the Invar and the fact that the heavy weight of the Invar appreciably increases the weight of the laser tube housing.
Another prior art approach is to flow the coolant across the flat surfaces, or through holes placed within the walls, of the metal laser tube housing. See U.S. Pat. No. 5,127,017, entitled “Electrical Excited Stripline Laser”, issued on Jun. 30, 1992 (Ref. 4); U.S. Pat. No. 7,260,134, entitled “Dielectric Coupled CO2 Slab Laser” issued on Aug. 21, 2007 (Ref. 5); and U.S. Pat. No. 7,263,116, entitled “Dielectric Coupled CO2 Slab Laser”, issued on Aug. 28, 2007 (Ref. 6).
In order to obtain sufficient cooling in accordance with the teachings of Refs. 4-6, the coolant needs to flow within holes placed within the thickness of the flat or tubular surfaces of the laser tube housing walls, as discussed in Refs. 4-6. This approach requires that the walls of the laser tube housing to be thicker in order to compensate for the loss of stiffness in the structure. The stiffness is required to minimize the pressure effects of the atmosphere deforming the partially evacuated laser tube housing (i.e., which contains a typical pressure between 50 Torr to 150 Torr) and, therefore, affecting the optical resonator alignment. This approach is not attractive because of increased weight and cost.
The present invention provides a superior approach to addressing this issue of laser tube housing cooling and stiffness.